Recently I have added a 10gb nic to my pc and connected it to sfp+ port on my rb4011 gs+rm router.
Via what medium? Is this LC-LC fiber, or Cat-7 copper, or what? Which SFP+ module are you using on the RB4011 end? If it isn't a copper connection, is it a matching SFP+ module on the PC end, or have you mixed-and-matched?
using bandwidth test
Do you mean the internal RouterOS bandwidth test or some other test?
The MikroTik test only works against another MikroTik router, not against the PC itself. There are some old "btest" type programs floating about for PC OSes, but they only work with password authentication turned off, as far as I know, and I'm not willing to do that just to run the test.
The RB4011 has only one 10G port, so if you're doing something like iperf3 from one PC to another across the RB4011, you can't possibly get 10G to the other device, because it'll bottleneck down to 1G at the GigE ports.
I've got a btest running right now between a CRS328 and an RB4011, and I get about 1200 Mbit/sec over OM4 MMC fiber, unidirectional. (Bidirectional runs slower, as a rule.)
Keep in mind that the RB4011 is a router, not a switch, and
its SFP+ cage is connected directly to the CPU, not to the switch chip. As a rule of thumb, you can only push about 1 Gbit/sec per 1 GHz of CPU power, so there you go with my 1.2 Gbit/sec results on this 1.4 GHz CPU. There are 4 cores and the internal btest has a connection count feature, but it appears the test isn't strongly multi-threaded:
/system resource cpu> print
# CPU LOAD IRQ DISK
0 cpu0 56% 10% 0%
1 cpu1 49% 11% 0%
2 cpu2 33% 10% 0%
3 cpu3 39% 4% 0%
To get something closer to the ideal 10G line speed, you'd want to run an external bandwidth test between high-powered hosts across 10G capable switches, not through a router's CPU. That means an arrangement like two PCs running iperf3 across two of the SFP+ ports of a CRS328 configured not to do any "routing" or clever L3 type stuff, with all of the hardware offloading features enabled. The last time I tried that with my CRS328, I believe I got something like 5G, not the full 10G over the same OM4 fiber.